Cathode for anodic protection system



Y 1967 J. F. DELAHUNT 3,317,415

CATHODE FOR ANODIC PROTECTION SYSTEM Filed June 20, 1963 2 Sheets-Sheet 1 FIGURE I I I Tronspclssivel A Region I I l l I l -O.5 --I.O l.5

POTENTIAL (Volts v. Pt Electrode) ACID INLET ACID DRAlN NOZZLE FIGURE 2 FIGURE 3 JOHN F. DELAHUNT INVENTOR BY W I y PATENT AGENT y 2, 1967 J. F. DELAHUNT 3,317,415

CATHODE FOR ANODIC PROTECTION SYSTEM Filed June 20, 1965 2 Sheets-Sheet 2 o 55 N R l v & M

k (R /l\ r 9 g 5 I {i 3; x ii a a i T Q s 5 m o I i i F a IL\ g 1 I Q 1 II n g T g I m I 'L n l JOHN F. DELAHUNT mvENToR BY PATENT AGENT United States Patent 3,317,415 CATHODE FOR ANODIC PROTECTION SYSTEM John F. Delahnnt, Florham Park, N.J., assignor to Esso Research and Engineering Company, a corporation of Delaware Filed June 20, 1963, Ser. No. 289,275 4 Claims. (Cl. 204-196) The present invention is concerned with an apparatus for use in the anodic passivation of metallic surfaces in contact with corrosive solutions. Particularly, this invention relates to an improved cathode for use in the application of anodic passivation techniques to the protection of the bayonet heaters which are subjected to the attack of hot corrosive solutions.

In its simplest terms, anodic protection is the process of passivating an active-passive metal by impressing an external current on the metal. Three electrodes are necessary for anodic protection; the specimen to be protected is made the anode; an inert metal or a properly sized corrosion-active metal is used for the cathode and an electrochemical half cell is used as a reference electrode. A potentiostatic device regulates the current from the cathode to the working electrode (anode) in such a manner that the potential difference between the reference electrode and the anode should be independent of the total cell current. The anode, cathode, and reference electrode are in contact with a conductive fluid.

Application of anodic passivation techniques to the protection of the bayonet heaters used in heating vessels and more particularly in acid concentrators requires the solution of detailed process problems. Because of the particular design of the heating vessels and acid concentrators wherein several rows of bayonet heaters extend into the vessel body from the outer periphery of the vessel body, cathode placement and design is an important factor for consideration. In the normal operation of an anodic passivation system for the protection of bayonet heaters, one or more of the bayonets are electrically connected as cathodes or one or more of the bayonet heaters are replaced by metallic members which serve as the cathodes. -In this type of cathode placement, it has been found that the bayonet heaters connected as anodes located in close proximity to the cathodes of the system are often subjected to severe corrosion despite the application of a protective current. This result is believed to occur because the total passivating current supplied to the cathode which is suificient to protect all of the anodes of the system tends to pass in straight lines to adjacently located anodes causing the potential of these anodes to be shifted from the desired passive potential range to the transpassive region of the metal. Within the transpassive region, the corrosion rate of the bayonet surfaces is materially increased and the service life of the bayonets diminishes as a result.

An important object of this invention is to provide an efiicient cathode assembly for use in the application of anodic passivation techniques to the protection of bayonet heaters used in heating vessels and particularly in acid concentrators.

Another object of this invention is to provide a cathode assembly applicable for use in heating vessels such as, for example, acid concentrators, which can be constructed of inexpensive steel or ferrous alloy materials rather than expensive acid resistant materials of construction.

A further object of this invention is to provide an electrode assembly for a system of anodic passivation which is simple in construction, which will have a long service life, and which can be economically manufactured.

Further objects of this invention will be apparent from the following description when considered together With the accompanying drawings which illustrate the inventron.

The present invention resides in the discovery of an anodic passivation system for diminishing the corrosion of bayonet heaters in contact with corrosive solutions. As hereinafter described, the invention employs a novel cathode assembly wherein unexpectedly :any desired amount of passivating current may be passed through the solution so as to passivate the bayonets while minimizing the possibility of bringing those bayonet surfaces most closely adjacent to the cathode into their transpassive region and thus accelerating their corrosion. More specifically, the present invention is directed to a novel cathode assembly wherein the electrically active surface of the cathode is rigidly supported within the body of the vessel so that its longitudinal axis is substantially in a plane perpendicular to the longitudinal axis of the bayonet heaters which extend into the liquor belt of the heating vessel and/or acid concentrator. When the active portion of the cathode is supported in this manner in a position within the vessel beyond the inner extremity of the bayonets, the development of regions of transpassivity on the surface of the bayonets is materially arrested.

Because of the particular construction of the commercial design of many heating vessels such as for example acid concentrators, cathode assemblies cannot be suspended from the roof of the vessel because of the presence of a brick vapor shell over the liquor belt of the heating vessel. A cathode assembly located on the bottom of the heating vessel or acid concentrator would be operative; however, this type of arrangement would pose the problem as found in using one or more of the bayonets as cathodes, in that, the bottom row of bayonets in the heating vessel would be subjected to the possibility of the formation of regions of transpassivity on their surfaces due to the proximity of the cathode located at the bottom of the heating vessel.

The electrically active surfaces of the cathode assembly can be constructed of a variety of materials. Platinum or platinum clad materials, because of their inertness to almost all types of corrosive mediums, can be effectively used to practice the instant invention. However, in the preferred embodiment of the instant invention, less costly materials such as carbon steel or the inexpensive ferrous alloys are utilized for cathode construction. Cathodes constructed from ordinary materials are protected from the chemical action of corrosive mediums by application of simultaneous anodic and cathodic passivation techniques. Cathodic passivation of the cathode surface is achieved by constructing the cathode of such a size that the total current supplied the cathode surface is sufiicient to anodically protect the anode and also cathodically protect the cathode. Complete simultaneous anodic and cathodic protection is achieved when the ratio of the surface area of the bayonet heaters connected as anodes to the surface area of the active cathode members is about equal to the ratio of the current density required for the cathodic protection of the cathode members to the current density required for the anodic protection of the bayonet heaters connected as anodes. The current densities necessary to anodically or cathodically protect metals in any particular corrosive environment can be readily determined by routine laboratory experimentation.

In order that the invention may be understood, it will now be described with reference to the accompanying drawings in which:

FIGURE 1 is an anodic polarization curve indicating the relationship between the current necessary to maintain passivation of the anode at various potential states of the anode as compared with a platinum reference electrode.

FIGURE 2 is a schematic representative of the cathode apparatus of the instant invention.

FIGURE 3 is a layout view of a 90 section of the liquor belt of a commercial design acid concentrator which illustrates bayonet heater and cathode placement.

FIGURE 4 is a vertical sectional view of the T-section of the preferred cathode assembly.

FIGURE 5 is a vertical sectional view of the flange section of the preferred cathode assembly.

Referring to the drawings in detail, FIGURE 1 illustrates an anodic polarization curve for Alloy 20 stainless steel (0.07% C, 0.75% Mn, 1% Si, 20% Cr, 29% N1, 2% Mo, 3% Cu, balance Fe) in 65% field H 80 at a metal temperature of 300 F. and an acid temperature of 260 F. The Alloy 20 steel-sulfuric acid system is illustrated by way of example and not by way of limitation, for the improved cathode of the instant invention can be utilized readily in acid concentrators and heating vessels generally which employ bayonet heaters constructed of any active-passive metal. Generally, most materials of construction including all types of ferrous alloys and stainless steels fall within this category. Referring to FIGURE 1, point A designates the normal state of Alloy 20 stainless steel in 260 F. field sulfuric acid. In actual operation of an anodic protection system, it has been observed that starting from this normal potential state of active-passive metal and progressing to more noble potentials, the current increases to a maximum value designated by point B. The potential value at this P011111 of maximum current is called the critical passivating potential. The maximum current value for Alloy 204-1 80.; systems rarely exceeds amps/cmf The critical passivating potential depends upon the temperatures of the system and the corrosive liquid being heated. At the point of critical passivating potential, corrosion of the metal surface is at a maximum value. As the potential is brought to more noble values, it is believed that an oxide film begins to form upon the surface of the metal until a point is reached within the passive potential range where a further change of potential usually results in a small or no further change of current density. Point C designates the beginning of the passive potential region. As the potential is increased to more noble or cathodic values, a point is reached where the passivating current again increases with increasing potential values. Pomt D designates the beginning of the transpassive region and oxygen evolution. In the anodic passivation of the bayonet heaters of heating vessels, improper cathode design and placement causes the bayonet surfaces located ad acent to the cathodes to be placed in the transpassive region causing their rate of corrosion to be materially 1ncreased.

Referring to FIGURE 2, reference numeral 10 designates the electrically active portion of the cathode assembly. In the embodiment shown in FIGURE 2, the electrically active members 10 are constructed of an electrically conductive material, preferably metallic such as for example, a standard plugged pipe, and are connected to and electrically insulated from T joint 20. The cathode members are rigidly positioned within the liquor belt of the concentrator or heating vessel by means of support structure 30 whose inner end is connected to T joint and whose outer end is connected to a standard blind flange 40 which is connected to the side walls of the vessel. When the cathode assembly is positioned in the manner contemplated by the instant invention, the metallic bayonet heaters are protected against corrosion even though the level of the corrosive solution in the vessel varies to an appreciable extent. It should be understood, however, that while the construction of the cathode assembly of the present invention has been described utilizing specific materials, metallic rods or bars may be used rather than the preferred pipe construction. Additionally, the active cathode members can 'be properly positioned within the vessel by one or more support members located at either the top or bot-tom of the tank.

FIGURE 3 illustrates the desired placement of the cathode assembly within an acid concentrator body. The acid concentrators widely used in the process industries are generally divided into four compartments. FIGURE 3 is a layout view of one compartment of such an acid concentrator having a plurality of rows of bayonet heaters 11 and in the particular embodiment of the present invention shown in FIGURE 3, a centrally located bayonet heater is removed from the acid concentrator and a cathode support member such as for example shown in FIGURE 2 and T joint 20 is inserted in its place. The cathode support structure is of such a length that the T joint 20 is located beyond the inner periphery of the bayonet heaters. The electrically active members 10 (cathodes) are then connected to the electrical conductors located within the T joint 20. When positioned as contemplated :by the instant invention, the cathodes 10 can if desired, extend above and below the top and bottom rows of bayonet heaters 11 of the acid concentrator. In a particular design of an acid concentrator commonly used in the industry, the total height of the active portion of the cathode assembly is about six to eight feet depending upon the number of rows and spacing of the bayonet heaters. In operation, the cathode assembly of the instant invention serves to facilitate the application of anodic passivation techniques to the protection of acid concentrator bayonet heaters 11 by providing an eflicient electrode which materially diminishes the formation of regions of transpassivity on the surface of the bayonets located most closely adjacent to the electrode assembly.

FIGURE 4 illustrates the electrical connections of one embodiment of the instant invention. Cathode members 10 which in a preferred embodiment are pipe structures are attached to the electrical conductors located within T joint 20 by means of bolt 21 which is attached to plug 22 which is threaded within the cathode members 10. Bolt 21 is threaded into conductor 23. The cathode members 10 are insulated from T joint 20 by means of bearing and washer 24 which can be constructed of any suitable type of material which will withstand the temperature, electrical and chemical environment in which the electrode assembly is being used. It has been found that a polytetrafluo-roethylene resin, sold under the registered trademark Teflon is particularly useful in a high temperature acid environment. The current flow of the system is carried by conductor 25 whose inner end is connected to conductor 23. The outer end of conductor 25, contained within the cathode support structure 30 extends to the side wall of the concentrator body wherein it is electrically connected to the negative pole of the potential control and power supply means of the system. The potential control and power supply means of the system generally consists of a potentiostatic device alone or in combination with an auxiliary direct current power source. In operation, the potentiostatic device is preset at a potential value U within the passive potential range of the anode being protected. This assigned predetermined potential value U is compared in the comparator circuits of the potentio-staticdevice with the existing potential U, as measured between the reference electrode and the bayonet heaters connected as anode. The difference dU=U U is amplified to V dU in the voltage amplifier of the system. This amplified difference potential controls the power amplifier. This instrument furnishes the current which flows from the cathode to anode which is necessary to make the actual potential value Uj equal to the assigned value U Conductors 23 and 25 can be constructed of any electrically conductive material; however, it has been found that brass rods can be easily adapted for service in the present invention. T joint 20, cathode supprot 30 and flange 40 (FIGURE 2) may be constructed of any environment insensitive material which can be immersed in hot corrosive solutions for long periods of time.

The electrical connections of the cathode assembly located at the wall of the vessel are illustrated in FIGURE 5. In the assembly shown in FIGURE 5 the cathode support member 30 is continuously welded to the corrosion resistant flange 40 at point 41. Electrical conductor 25 which extends through support member 30 passes through flange 40 at its outer end at point 42. The electrical conductor 25 which is threaded at its outer extremity is electrically insulatedv from flange 40 and support nut 44 by means of a micarta bushing and washer assembly 43. The current flow to conductor rod 25 is carried from the potential control and current supply means of the system by means of electrical contact 45 and conducting means 47 which is securely attached to the terminal of the contact. The electrical contact 45 is maintained in position on the conductor rod by means of support nuts 44 and 46. The electrical connections located on the outer surface of flange 40 are protected from the elements by means of a weatherproof housing 48.

The following exmaple illustrates a preferred design of the cathode assembly for commercial design acid concentrators.

Example 1 In the application of anodic passivation techniques to the protection of the bayonet heaters of acid concentrators, the cathode assembly consists of a cathode support structure of the type shown in FIGURE 2 constructed of a standard two inch Schedule 80 acid resistant pipe, a standard two inch acid resistant T joint, and two platinum clad, two inc-h Schedule 80 plugged pipes. To insure satisfactory operation of the cathode assembly, the cathode support structure extends 6 to 18 inches beyond the inner periphery of the six foot bayonet heaters, and the electrically active cathode members attached to said sup port structure extend vertically two to three inches above and below the top and bottom rows of bayonet heaters. The total height of the cathode members is about eight feet. In operation, the cathode members are electrically connected to the negative terminal of the potential control and power supply means of the protection system. The plurality of bayonet heaters as anodes are connected in parallel and further connected tothe positive terminal of the potential control and power supply means of the system. Direct current energy is supplied to the bayonet heaters as anodes by way of the cathode in amounts sufficient to p-assivate the anodes and to maintain the average potential of the bayonet heaters within their passive potential range With respect to a reference electrode in electrochemical contact with the sulfuric acid solution. Reference electrodes of any suitable type may be used in the practice of the instant invention. C-alomel and silver-silver chloride electrodes can be utilized in connection with salt bridges which electrically connect the electrode with the corrosive solution. Platinum reference electrodes are particularly adaptable or use in hot corrosive environments in that they can be directly immersed in the hot corrosive solutions.

While the present invention has been described with reference to the protection of the bayonet heaters of acid concentrators, the cathode assembly may be advantageously used in any corrosive system which employs the use of a plurality of bayonet heaters to supply heat requirements such as, for example, bayonet heaters used in storage equipment to maintain desired fluid temperatures, and bayonet heaters used in fractionation operations where fractionation heat requirements are supplied :by bayonet heaters located in the bottoms section of the fractionation column.

Resort may be had to various modifications and variations of the invention without departing from the spirit of the discovery or the scope of the appended claims.

What is claimed is:

1. An apparatus for heating corrosive solutions comprising in combination a vessel having at least side Walls and a bottom wall, one of said side walls including a plurality of heater receiving apertures, a plurality of substantially parallel elongated bayonet heaters formed out of an active-passive metallic material extending through respective ones of said apertures perpendicular to the side walls of said vessel into said vessel, at least one reference electrode adapted to be mounted in said vessel in electrical contact with said corrosive solution, at least one potentiostatic device suitable for the supply of direct current energy and for the maintenance of a predetermined potential between said bayonet heaters and reference electrode, a tubular cathode support structure formed out of an environment insensitive material extending through an unused one of said heater receiving apertures, said cathode support structure extending substantially parallel to said heaters and having an inner end and an outer end, said outer end being connected to and supported by the side wall of said vessel, said inner end extending into the vessel beyond the inner periphery of said bayonet heaters, a hollow T joint formed out of environment insensitive material and having its stem connected to the inner end of the cathode support structure and its cross-bar extending substantially perpendicular to the axis of said heaters, a pair of cathode members connected to each end of the T crossbar and electrically insulated from the T joint, the longitudinal axis of said cathode members extending in a plane substantially perpendicular to the longitudinal axis of the bayonet heaters, a conductor rod having an inner end and an outer end, said conductor rod extending through and electrically insulated from said cathode support structure having its outer end extending through and electrically insulated from the side walls of said vessel and its inner end extending into said T joint, means to electrically connect each of said cathode members to said conductor rod, electric-a1 conduction means connecting the positive terminal of said potentiostatic device to the bayonet heaters as anodes and the negative terminal of said potentiostatic device to the outer end of said conductor rod and electrical conduction means connecting said reference electrode to said potentiostatic device.

2. The apparatus of claim 1 wherein said cathode members are cladded with platinum.

3. The apparatus of claim 1 wherein the ratio of the surface area of the bayonet heaters as anodes to the surface area of the cathode members is about equal to the ratio of the current density required for the cathodic protection of the cathode members to the current density required for the anodic protection of the bayonet heaters connected as anodes.

4. An apparatus for heating corrosive solutions comprising in combination, a vessel having si-de wlals and a bottom wall, one of said side walls including a plurality of heater receiving apertures, a plurality of substantially parallel elongated bayonet heaters formed out of an active-passive metallic material extending through respective ones of said apertures perpendicular to the side walls of said vessel into said vessel, a tubular cathode support structure formed out of an environment insensitive material extending through an unused one of said heater receiving ape1tures, said cathode support structure extending substantially parallel to the axis of said heaters and having an inner end and an outer end, said outer end being connected to and supported to the side wall of said vessel, said inner end extending into the vessel beyond the ends of the said bayonet heaters, a hollow T joint formed out of environmental insensitive material having its stern connected to the inner end of the cathode support structure and its cross-bar extending substantially perpendicular to said heaters, a pair of cathode members connected to each end of the T cross-bar and electrically insulated from the T joint, the longitudinal axis of said cathode members extending in a plane substantially perpendicular to the longitudinal axis of the bayonet heaters, a conductor rod having an inner end and an outer end, said conductor rod extending through and electrically insulated from said cathode support structure having its outer end extending through and electrically insulated from the side Walls of said vessel and its inner end extending into said T joint, means extending into said T joint electrically connecting each of said cathode members to said conductor rod, and electrical means connected to the outer end of said conductor rod for supplying current from the negative terminal of a potentiostatic device to said cathode members to thereby electrolytically protect said bayonet heaters as anodes in said heating apparatus.

1,366,257 1/1921 Gush 204-196 2/1910 Cumberland 204-196 15 8 Gunderson 204-496 Smith 204147 Sawyer et al 204-196 Bogg-s et al. 204 -196 Fischer 204-196 Fischer 204196 Balis 204-196 Locke 204-196 OTHER REFERENCES Edeleanu, Metallurgia, September 1954, pp. 113416.

Sudbury et al., Corrosion, vol. 16, No, 2, February 1960, pp. 471-541.

JOHN H. MACK, Primary Examiner.

T. H. TUNG, Assistant Examiner. 

1. AN APPARAATUS FOR HEATING CORROSIVE SOLUTION COMPRISING IN COMBINATION A VESSEL HAVING AT LEAST SIDE WALLS AND A BOTTOM WALL, ONE OF SAID SIDE WALLS INCLUDING A PLURALITY OF HEATER RECEIVING APERTURES, A PLURALITY OF SUBSTANTIALLY PARALLEL ELONGATED BAYONET HEATERS FORMED OUT OF AN ACTIVE-PASSIVE METALLIC MATERIAL EXTENDING THROUGH RESPECTIVE ONES OF SAID APERTURES PERPENDICULAR TO THE SIDE WALLS OF SAID VESSEL, AT LEAST ONE REFERENCE ELECTRODE ADAPATED TO BE MOUNTED IN SAID VESSEL IN ELECTRICAL CONTACT WITH SAID CORROSIVE SOLUTION, AT LEAST POTENTIOSTATIC DEVICES SUITABLE FOR THE SUPPLY OF DIRECT CURRENT ENERGY AND FOR THE MAINTENANCE OF A PREDETERMINED POTENTIAL BETWEEN SAID BAYONET HEATERS AND REFERENCE ELECTRODE, A TUBULAR CATHODE SUPPORT STRUCTURE FORMED OUT OF OF AN ENVIRONMENT INSENSITIVE MATERIAL EXTENDING THROUGH AN UNUSED ONE OF SAID HEATER RECEIVING APERTURES, SAID CATHODE SUPPORT STRUCTURE EXTENDING SUBSTANTIALLY PARALLEL TO SAID HEATERS AND HAVING AN INNER END AND AN OUTER END, SAID OUTER END BEING CONNECTED TO AND SUPPORTED BY THE SIDE WALL OF SAID VESSEL, SAID INNER END EXTENDING INTO THE VESSEL BEYOND THE INNER PERIPHERY OF SID BAYONET HEATERS, A HOLLOW T JOINT FORMED OUT OF ENVIRONMENT INSENSITIVE MATERIAL AND HAVING ITS STEM CONNECTED TO THE INNER END OF THE CATHODE SUPPORT STRUCTURE AND ITS CROSS-BAR EXTENDING SUBSTANTIALLY PERPENDICULAR TO THE AXIS OF SAID HEATERS, A PAIR OF CATHODE MEMBERS CONNECTED TO EACH END OF THE T-CROSSBAR AND ELECTRICALLY INSULATED FROM THE T JOINT, THE LONGITUDINAL AXIS OF SAID CATHODE MEMBERS EXTENDING IN A PLANE SUBSTANTIALLY PERPENDICULAR TO THE LONGITUDINAL AXIS OF THE BAYONET HEATERS, A CONDUCTOR ROD HAVING AN INNER END AND AN OUTER END, SAID CONDUCTOR ROD EXTENDING THROUGH AND ELECTRICALLY INSULATED FROM SAID CATHODE SUPPORT STRUCTURE HAVING ITS OUTER END EXTENDING THROUGH AND ELECTRICALLY INSULATED FROM THE SIDE WALLS OF SAID VESSEL AND ITS INNER END INTO SAID T JOINT, MEANS TO ELECTRICALLY CONNECT EACH OF SAID CATAHODE MEMBERS TO SAID CONDUCTOR ROD, ELECTRICAL CONDUCTION MEANS CONNECTING THE POSITIVE TERMINAL OF SAID POTENTIOSTATIC DEVICE TO THE BAYONET HEATERS AS ANODES AND THE NEGATIVE TERMINAL OF SAID POTENTIOSTATIC DEVICE TO THE OUTER END OF SAID CONDUCTOR ROD AND ELECTRICAL CONDUCTION MEANS CONNECTING SAID REFERENCE TO SAID POTENTIOSTATIC DEVICE. 